The present invention relates to material technology for lithium secondary batteries which can be used as power source for various devices such as computer, portable or cellular telephone, and portable audio device, and more specifically to positive electrode material for lithium secondary batteries and lithium secondary batteries having such positive electrode material.
As positive electrode material for lithium secondary batteries, there are known composite oxides, such as LiCoO2 and LiNiO2, including Li and transition metal such as cobalt, nickel and manganese.
Published Japanese Patent Specifications, Kokai Nos. H10(1998)-116614 and H11(1999)-71115 disclose spinel type lithium manganese oxides as positive electrode material.
The positive electrode material as disclosed in the above-mentioned Japanese Published Specifications has a structure to improve the durability by the substitution, for lithium or manganese in lithium manganese oxide, of another metal. Though such a structure is advantageous in durability, it is not competent enough to prevent undesired decrease in charge-discharge capacity of a battery, especially in initial charge-discharge capacity.
It is therefore an object of the present invention to provide positive electrode material for a lithium secondary battery and a lithium secondary battery having such positive electrode material capable of improving the lifetime of a lithium secondary battery without decreasing the charge-discharge capacity.
According to the present invention, a positive electrode material for a lithium secondary battery comprises: a lithium manganese oxide having a spinel structure, expressed by one of the general formulae;
LixMnyO4 
(where 1xe2x89xa6xxe2x89xa61.33 and 3xe2x88x92x less than yxe2x89xa63.1xe2x88x92x); and
LixMnyMzO4 
(where M is a metallic element other than Li and Mn, 1xe2x89xa6xxe2x89xa61.33, 3xe2x88x92xxe2x88x92z less than yxe2x89xa63.1xe2x88x92xxe2x88x92z, and O less than zxe2x89xa61.0).
The metallic element M may be at least one selected from the group consisting of Mg, Al, Cr and Ni.
A lithium secondary battery according to the present invention comprises: a negative electrode comprising a negative electrode active material capable of retaining and releasing lithium ions; a nonaqueous electrolyte having ionic conductivity of lithium ions; and a positive electrode comprising a positive electrode active material comprising a composite oxide which comprises Li and which is capable of retaining and releasing lithium ions. The composite oxide is a lithium manganese oxide having a spinel structure expressed, by one of the above-mentioned general formulae.